As the wireless communications market continues to expand, the accompanying need for increased capacity is forcing a move from analog modulation techniques, such as frequency modulation (FM), to digital modulation formats, such as time division multiple access (TDMA) and code division multiple access (CDMA), which have bandwidths listed in Table 1.
TABLE 1 US DAMPs (TDMA) 30 kHz GSM 277 kHz CDMA 1.23 MHz
Both TDMA and CDMA modulation require somewhat greater linearity than can be routinely obtained in an uncorrected, high efficiency class AB power amplifier. Unfortunately, conventional correction mechanisms, such as feed forward or predistortion schemes, are complex, inefficient, and prohibitively expensive to be practical solutions for correcting distortion in the majority of single carrier linear power amplifiers. Since baseband I and Q data is usually not available at the power amplifier, baseband correction techniques which make use of baseband signals, such as baseband cartesian feedback, are also not applicable.